The present invention relates to the field of ferrimagnetic resonators and more particularly to the geometry of the physical design of the pole pieces of the electromagnets used in ferrimagnetic resonators so that higher frequency operation can be achieved.
Magnetically tunable filters and oscillators that employ ferrite resonators such a Yttrium-Iron-Garnet (YIG) have for many years used tuning magnets with relatively long cylindrical poles having beveled tips. FIG. 1 shows such a prior art tuning magnet.
In order to minimize hysteresis, the magnet is usually made from an alloy of nickel and iron in approximately equal proportions. However, when this alloy is used with the prior art magnet geometry described above, tuning linearity is lost at frequencies above 20 GHz.
Tuning to higher frequencies with the prior art geometry can be accomplished by using an alloy that can accommodate a higher magnetic flux density. Substituting a cobalt and iron alloy for the nickel and iron alloy permits greater flux densities, and therefore higher frequency operation, but the cobalt and iron alloy has approximately twenty times as much hysteresis as the nickel and iron alloy does. For many applications, this level of hysteresis is entirely unacceptable because the relationship between the current applied to the coils and the resulting frequency becomes difficult to predict, since it depends on the history of prior operation as well as the current presently being applied.
Part of the problem with alternative alloys and the extra hysteresis that they create can be solved by the use of limited quantities of the high hysteresis alloy at the most critical parts of the magnetic pole construction. A small layer of the high hysteresis alloy is used at the tip of the pole to minimize saturation effects, while the rest of the pole is made of a lower hysteresis alloy that saturates more easily. This approach permits an effective trade-off between the two competing goals of maximum magnetic flux density and minimum hysteresis.
Ferrimagnetic resonators have traditionally used a relatively long, cylindrical pole shaft terminating in a beveled tip, with the tip region being much shorter than the pole shaft. Alternative pole geometries have been used in the design of large-scale, high-power magnets, especially in the field of very high power magnets used in research and high energy physics. But these relatively exotic pole geometries have never been applied to the field of ferrimagnetic resonators, such as those employed in YIG filters and oscillators.
What is desired is an electromagnet geometry for ferrimagnetic resonators that will produce higher magnetic flux densities at the tip, without a significant penalty in terms of increased hysteresis, permitting higher frequency operation of the ferrimagnetic resonators.